The present invention relates to a synchronous receiving method employed for receiving a signal train including synchronizing signals, such as selective calling signals.
A signal transmitted from the transmitting side in a transmission system of selective calling signal compreises a first code of a predetermined format which enables a receiving unit to easily detect that information is being sent over a channel, a second code of a predetermined format which succeeds the first code and causes the receiving unit to be synchronized with the information transmitted over the channel, and a third code which succeeds the second code and sends information to the receiving unit.
The receiving unit can be synchronized with the transmitted signal by detecting again the second code of a predetermined format as a synchronizing code, as mentioned above. Conversely, where the second code is not detected once or more times in succession as predetermined, control for battery saving before establishment of synchronization is effected again by which the receiving unit performs intermittent reception. A method which involves repeated detection of the second code of synchronizing code for acquiring synchronization possesses disadvantages such as follows:
(1) The battery saving efficiency is low because it cannot be decided, until the detection of the second code of a predetermined format, whether or not information is being provided on the channel.
(2) Since a portion of information is alloted to the second code, the amount of information which can freely be used is small.
(3) Where it is desired to send a large quantity of information at a higher transmission speed than at present, the time for sending the information increases.
(4) The battery saving efficiency is low because it is necessary to supply power to the receiving section and the decoder for synchronization at the timing for sending out the second code.